The chosen interval between wire locators guarantees that the sagitta due to
electrostatic forces on a not perfectly centered anode wire remains small.
Typically, these are spaced at intervals of 80 cm.
The precise distance necessarily depends on the
total wire length to be subdivided and thus varies slightly from
module to module.
The largest number of locators per wire section,
i.e. between end-and middle-block, is two.

We experimented with locators of quite different shapes.
The final choice is suitable for production by injection moulding
and was designed by
NIKHEF
in cooperation with
Philips High Tech Plastics.

The design was based on the following criteria:

minimization of amount of material and length of the locator:
the locator is executed in Noryl N110 (suitable for injection moulding)
and represents 0.45% X0, averaged over its length and width

minimization of the number of moving parts in the mold, in order to
maximize the accuracy of the wire centering in the straw tube

ease of wire insertion through a series of locators
(i.e. time needed for the operation and failure rate)

The locator essentially consists of two disks from which V-shaped sections,
with opposite orientations, have been cut out, the overlap of which
leaves an opening of about 0.050mm diameter for the anode wire.